1. Field
The present invention relates to a cylinder lubricant for a two-stroke marine engine, which can be used with both high-sulphur content fuel oils and low-sulphur content fuel oils. It relates more particularly to a lubricant having a sufficient neutralizing power vis-à-vis sulphuric acid formed during the combustion of high-sulphur fuel oils, whilst limiting the formation of deposits during the use of low-sulphur fuel oils.
2. Technological Background of the Invention
The marine oils used in 2-stroke slow-speed crosshead engines, are of two types. Cylinder oils on the one hand, ensuring the lubrication of the cylinder piston assembly, and system oils on the other hand, ensuring the lubrication of all the moving parts outside the cylinder piston assembly. Within the cylinder piston assembly, the combustion residues containing acid gases are in contact with the lubricant oil. The acid gases are formed from the combustion of the fuel oils; they are in particular sulphur oxides (SO2, SO3), which are then hydrolyzed during contact with the moisture content present in the combustion gas and/or in the oil. This hydrolysis produces sulphurous (HSO3) or sulphuric (H2SO4) acid.
In order to preserve the surface of the jackets and avoid excessive corrosive wear, these acids must be neutralized, which is generally carried out by reaction with the basic sites included in the lubricant. The neutralizing capacity of an oil is measured by its BN or Base Number, which characterizes its basicity. It is measured according to the standard ASTM D-2896 and is expressed in equivalents by weight of potash per gram of oil or mg KOH/g. The BN is a standard criterion making it possible to adjust the basicity of the cylinder oils to the sulphur content of the fuel oils used, in order to be able to neutralize all of the sulphur contained in the fuel, and capable of being converted to sulphuric acid by combustion and hydrolysis. Thus, the higher the sulphur content of a fuel oil, the higher the BN of a marine oil must be. This is why BNs varying from 5 to 100 mg KOH/g are to be found on the marine oils market.
Environmental concerns have led, in certain areas and in particular coastal areas, to requirements for the limitation of the sulphur level in the fuel oils used on ships. Thus, the regulation MARPOL Annex 6 (Regulations for the Prevention of air pollution from ships) of the IMO (International Maritime Organization) entered into force in May 2005. It provides for a maximum sulphur content of 4.5% m/m of the heavy fuel oils as well as the creation of sulphur oxide emission control areas, called SECAs (SOx Emission Control Areas). Ships entering these areas must use fuel oils with a maximum sulphur content of 1.5% m/m or any other alternative treatment aimed at limiting SOx emissions in order to comply with the values specified. The notation % m/m denotes the mass percentage of a compound relative to the total weight of fuel oil or lubricant composition in which it is included.
Ships sailing trans-continental routes will then use several types of heavy fuel oil depending on local environmental constraints, allowing them to optimize their operating costs. Thus the majority of container ships currently under construction provide for the utilization of several bunker tanks, for a “high sea” fuel oil with a high sulphur content on the one hand and for a ‘SECA’ fuel oil to with a sulphur content less than or equal to 1.5% m/m on the other hand.
Switching between these two categories of fuel oil can require adaptation of the engine's operating conditions, in particular the utilization of appropriate cylinder lubricants. Currently, in the presence of fuel oil with a high sulphur content (3.5% m/m and more), marine lubricants having a BN of the order of 70 are used. In presence of a fuel oil with a low sulphur content (1.5% m/m and less), marine lubricants having a BN of the order of 40 are used. In these two cases, a sufficient neutralizing capacity is achieved as the necessary concentration in basic sites provided by the overbased detergents of the marine lubricant is reached, but it is necessary to change lubricant at each change of type of fuel oil. Moreover, each of these lubricants has limits of use resulting from the following observations: the use of a cylinder lubricant of BN 70 in the presence of a fuel oil with a low sulphur content (1.5% m/m and less) and at a fixed lubrication level, creates a significant excess of basic sites (high BN) and a risk of destabilization of the micelles of unused overbased detergent, which contain insoluble metallic salts. This destabilization results in the formation of deposits of insoluble metallic salts (for example calcium carbonate), mainly on the piston cover, and can eventually lead to a risk of excessive wear of the jacket-polishing type.
Therefore, the optimization of the cylinder lubrication of a slow-speed two-stroke engine then requires the selection of the lubricant with the BN adapted to the fuel oil and to the operating conditions of the engine. This optimization reduces the flexibility of operation of the engine and requires a significant degree of technical expertise on the part of the crew in defining the conditions under which the switching from one type of lubricant to the other must be carried out. In order to simplify the operations, it would therefore be desirable to have a single cylinder lubricant for two-stroke marine engines which can be used with both high-sulphur content fuel oils and the low-sulphur content fuel oils.